1. Field of the Invention
The present invention relates to a coating method, and particularly relates to a method of wet coating for applying an anti-reflective film to a substrate.
2. Description of Prior Art
It is well known that, all uncoated, optically transparent materials reflect a portion of incident light. To increase the light transmission and thus reduce the reflection loss, appropriate interference layers are applied. The surface to be subjected to antireflection treatment is coated with one or several thin layers having a suitable refractive index and an appropriate thickness. The structure of the interference layer is so configured that destructive interference phenomena occur in the reflected radiation field in suitable wavelength ranges, so that reflexes from light sources are strongly reduced in terms of their brightness. A single quarter-wavelength coating of optimum index can eliminate reflection at one wavelength. Multi-layer coatings can reduce the loss over the visible spectrum.
Two widely used anti-reflective (AR) coating methods are physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD includes methods of evaporation, sputtering, molecular beam epitaxy, and vapor phase epitaxy. PVD operates by changing a solid material to a vapor in a deposition chamber and allowing the vapor to condense on a substrate. However, in PVD process, the interface between the substrate and film is generally distinct, which in many cases produces poor relative adhesion, and thus may induce frilling and even peeling of the film. With the continuing trend of an optical substrate toward small size and large curvature, the possibility of frilling and even peeling of the film formed by the PVD process from the substrate increases, since the surface effect increases with reduced size and increased curvature. Further, the implementation of PVD process requires complicated apparatus. The high cost to purchase, operate, and maintain the apparatus, restricts its application to central production facilities. The evaporative method also causes heating of the substrate because convective cooling is inefficient in a vacuum and the hot elemental materials emit thermal radiation that may be absorbed by the substrate. The heating can cause substrate damage, such as internal stress and warping, especially with plastic substrates.
CVD process is another deposition process that produces a non-volatile solid film on a substrate by the surface pyrolized reaction of gaseous reagents that can contain the desired film constituents. However, the CVD process is carried out at a higher temperature than PVD, and typically heats the substrate in a range from 500 degrees Centigrade to 1600 degrees Centigrade. Such high temperature deposition causes progressive interfacial diffusion and the formation of pores, fractures and the intermetallic phase in the interfacial region, thereby significantly decreasing film adhesion. Therefore, the main disadvantage of the CVD process is the relatively high substrate temperature that is needed. High temperature in general requires that the coefficient of thermal expansion of the substrate and the coating must be matched, or else excessive strains are introduced into the coating. This is particularly advantageous in the coating of temperature-sensitive plastic substrates, thereby limiting the kind of substrates that can be coated by CVD process.
The sol-gel method is also known as an AR film coating method. Sol-gel is a useful method for the deposition coatings of oxide materials. Pre-cursors containing the element whose oxide is to be deposited are reacted in a solvent medium with water. Hydrolysis and condensation reactions proceed to generate colloidal dispersions of the oxide. These dispersions can be used to deposit oxide coatings by dip, spin or spray coating methods. Once deposited onto a substrate, thermal treatment of the substrate converts the colloidal particles into a compact mechanically sound oxide layer. Since thermal treatment is required, this method is also not suitable for temperature-sensitive plastic substrates. Furthermore, with the dip coating process, there is a tendency that coating solution flows down due to gravity after the substrate is withdrawn from the coating bath, which makes the thickness gradually increase from the top to the bottom. This is known as a coating “wedge”. This is especially noticeable in the case where the coating solution has very low viscosity and the coating process has high withdrawal speed. Related dip coating methods and apparatus are disclosed in U.S. Pat. Nos. 5,693,372; 5,681,391; 5,578,410; 5,633,046 and 5,965,210, the disclosures of which are incorporated herein by reference.
It is, therefore, desirable to provide a method for coating AR films with uniform thickness and strong adhesion that only requires compact, inexpensive device and can be performed at normal temperature to avoid heat damage.